As the technology of electronic digital storage devices improves, the application of such devices to everyday uses has greatly increased. One such application is a circuit using digital electronic storage devices to function as a digital telephone answering device (DTAD). Many of today's digital telephone answering devices use dynamic ram (DRAM) or audio ram (ARAM) as the electronic storage medium for storing data associated with DTAD systems. One characteristic of these types of electronic storage devices is that the memory locations containing data within these devices continually needs to be refreshed. If these memory locations are not refreshed within a given amount of time, the data contained within these memory locations will be lost.
An important feature of the DTAD relates to its power consumption during a power failure, when the DTAD is operating from battery power. In this mode of operation, the primary objective of the DTAD is to prevent the loss of messages stored in the DRAM memory devices. To achieve this objective, the DTAD must reduce its power consumption in order to extend the life of the battery, which is the only source of power available at that time. Thus, to reduce the power consumption of the DTAD while operating from battery power, the only functions which are active are those which continually refresh the data stored within the DRAM memory devices. As a result, nearly all of the battery power consumed during a power failure goes to refreshing the DRAM memory devices.
However, a common problem of the DTAD is its loss of data due to an extended power failure. This loss of data occurs as a result of the battery life of the battery expiring. The loss of battery life is in turn due to the amount of battery power required to refresh the DRAM memory devices. As stated previously, nearly all of the battery power consumed during a power failure goes to refreshing the DRAM memory devices. The power consumed during these refresh operations depends upon the number of memory locations which need to be refreshed, and upon how often these memory locations need to be refreshed.
In a conventional DTAD, a plurality of DRAMs are grouped together to form a large memory block wherein the message data is stored. This block of DRAMS is referred to as a DRAM memory structure. Within each DRAM is an array of memory locations organized by rows and columns. During the refresh operation, all memory locations having the same row address in each of the DRAMs are refreshed simultaneously. The refresh operation commences refreshing at the row with the lowest row address in each of the DRAMs, and continues refreshing, row-by-row, each succeeding row in each of the DRAMs simultaneously until the row with the highest address in each of the DRAMs has been refreshed.
This systematic procedure results in the refreshing of every memory location within each of the DRAMs regardless of whether these memory locations contain or do not contain data. If 100% of the DRAM memory structure is filled with data, then this refreshing scheme is an efficient way of refreshing the data contained within the DRAMs. However, this is not usually the case. Typically the DRAM memory structure of a DTAD is 10%-25% filled to capacity, which means that the refreshing scheme described above will needlessly consume excess power to refresh memory locations which contain no data. During a power failure, the refreshing of memory locations which contain no data results in the unnecessary waste of battery power, thereby reducing the battery life of the battery.
Thus, the power consumption of digital telephone answering devices has not proved to be as efficient as desired, particularly during extended power failures where an extended battery life time is critical to the preservation of messages stored within the devices' electronic memory structures. It is therefore an objective of the present invention to reduce the power consumption of conventional digital telephone answering devices.